Copending U.S. patent application Ser. No. 394,959 (filed on July 2, 1982) now abandoned discloses composite overhead stranded conductor of this type, an example of which is shown in FIG. 1 herein. In FIG. 1, an optical unit 5 is composed of a plurality of optical fibers 2 arranged in a corresponding number of helical grooves 1 formed in the outer surface of an aluminum spacer member 3 surrounded by an outer tube 4 also made of aluminum. The outer surface of the optical unit 5 is surrounded by a stranded conductor layer composed of a plurality of aluminum-clad steel wires 6.
The composite stranded conductor shown in FIG. 1 exhibits an acceptable mechanical strength and excellent protective characteristics against lightning due to the fact that the optical unit 5 is received in the outer tube 4, which performs a protective function.
If the outer diameter of the optical fiber 2 is sufficiently smaller than both the width and depth of the helical groove 1 to be received therein loosely, compression and/or expansion stresses applied thereto are absorbed and/or weakened due to radial movement of the optical fibers 2 in the grooves 1, such as shown by thick arrows in FIG. 2. In the structure shown in FIG. 1, however, if the width of the groove 1 of the spacer 3 is much larger than the diameter of the optical fiber 2, bending and/or compression stresses applied to the composite stranded conductor cause the optical fiber to move not only radially but also circumferentially, resulting in irregular bending of the optical fibers. Accordingly, transmission loss of the optical fiber may increase.